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Wouldn't a droplet of liquid (say water) floating around in space become spherical and "in hydrostatic equilibrium" due to surface tension but not gravity?

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    $\begingroup$ The title has nothing to do with the question. $\endgroup$ – Rob Jeffries Dec 4 '14 at 0:41
  • $\begingroup$ @RobJeffries I edited it a little to make it more descriptive. $\endgroup$ – HDE 226868 Dec 4 '14 at 1:22
  • $\begingroup$ It is important to state if the droplet is confined by a surrounding fluid (e.g. air) which exerts a pressure upon it. Without such a fluid the molecules of a liquid like water will quickly "boil off" i.e. fly away due to their unrestricted thermal motions. Also you need to indicate what gravitational field applies at the position in space. Surface tension is a result of net attractive inter-molecular attractions (e.g. van der Waals forces). $\endgroup$ – steveOw Dec 4 '14 at 15:01
  • $\begingroup$ @RobJeffries: The gist of my question has been edited away. As is stands now, this is a prerequisite for my original question, but I don't see why you thought the title and question didn't match.... $\endgroup$ – ThePopMachine Dec 4 '14 at 15:14
  • $\begingroup$ @PopMachine The title was a little unclear. Were you asking if the definition of a planet should include gravity and/or hydrostatic equilibrium? Or something else. At any rate, you can and should roll it back or edit it if my edit was wrong. $\endgroup$ – HDE 226868 Dec 4 '14 at 16:07
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Strictly speaking (as far as I know), hydrostatic equilibrium applies whenever a fluid balances external body forces with the pressure gradient. From Wikipedia:

In continuum mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure gradient force.

I think the concept happens to be most frequently used in areas where gravity is the external force, but it could in principle be anything else. So, though I stand to be corrected, I think a droplet isolated in space long enough could be said to be in hydrostatic equilibrium, even though the most relevant force is the surface tension, rather than gravity.

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Video: Space Station Astronauts Grow a Water Bubble in Space

Surface tension tends to draw the water into a nice sphere.

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Yes, a droplet of water would do that.

However, at the planetary scale, the contribution of superficial tension is negligible. At that scale, gravity dominates all other forces.

In fact, you could model a planet as a loose mass or rubble, kept together by gravity alone, and the model would be very close to reality.

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We have found instances of asteroids that are mostly rubble piles, but spinning fast enough that they would fly apart if bound only by gravity. The most likely binding force is static charge, although some sort of glue is not out of bounds. I don't know if either of these would be called "hydrostatic", but if the question is really "Is gravity the only force that can bind a non-solid object together?" the answer is no.

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